Dye-sensitized solar cells (DSSCs) are currently attracting great interest owing to their low production costs and high energy conversion efficiencies to be a potential alternative to conventional inorganic photovoltaic devices.
The DSSCs are typically based on a metal oxide semiconductor layer, in particular a TiO2 semiconductor layer, coated with a dye layer, which is in contact with a redox electrolyte.
By using ruthenium complex sensitizers and liquid electrolytes, high energy conversion efficiencies have been reported for DSSCs. However, the presence of a liquid electrolyte in DSSCs triggers several problems including the leakage and evaporation of the liquid solvent, the possible desorption of the attached dyes and the corrosion of the counter electrodes, which limit the long-term performance and practical use of the DSSCs.
Several methods have been proposed to reduce the evaporation and leakage of the liquid electrolyte by using solid or gel materials in substitution for the liquid electrolyte. The main alternatives include gel materials incorporating redox couples. However, the ion diffusion and conductivity characteristics of the solid and gel electrolytes are usually less than that of a liquid electrolyte.
Fluoropolymers, in particular vinylidene fluoride polymers, have been used as raw materials for polymer gel electrolytes because of their superior properties in terms of ionic conductivity and thermal stability.
Nevertheless, polymer gel electrolytes might not incorporate and retain liquid plasticizer/liquid electrolyte in an effective manner during both manufacturing of the cell and operation of the same and/or might not possess suitable mechanical properties as required for effective separation of the electrodes.
On the other side, hybridization of organic and inorganic compounds is an important and evolutionary way to create polymeric compounds having, notably, enhanced mechanical properties. To elaborate such organic-inorganic polymer hybrids, sol-gel processes using metal alkoxides is the most useful and important approach. By properly controlling the reaction conditions of hydrolysis and polycondensation of metal alkoxydes, in particular of alkoxysilanes (e.g. tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS)), in the presence of pre-formed organic polymers, it is possible to obtain hybrids with improved properties compared to the original compounds.
Within this scenario, WO 2011/121078 (SOLVAY SOLEXIS S.P.A) 6 Oct. 2011 discloses certain fluoropolymer-based hybrid organic/inorganic composites obtained by a process involving the reaction of certain functional fluoropolymers possessing hydroxyl groups with certain hydrolysable compounds of Si, Ti, or Zr, and subsequent hydrolysis and polycondensation of the compounds so obtained. This patent document also mentions that the so obtained hybrid organic/inorganic composites can be notably used for various applications including manufacture of membranes for electrochemical applications. Thus, certain embodiments have been exemplified in such patent document, wherein films made by casting of the mentioned hybrid organic/inorganic composites were swelled with an electrolyte solution comprising a solvent (mixture of ethylene carbonate and propylene carbonate) and an electrolyte (LiPF6).
In order to ensure appropriate workability during fluoropolymer-based hybrid composite manufacture and/or during casting of the same, it is current practice to use organic solvents such as N-methyl-2-pyrrolidone (NMP), possibly in admixture with other solvents.
Nevertheless, the use of NMP is attracting more and more concerns, having regards to the safety risks associated to its handling and to possible leakage/emissions in the environment. NMP has been notably classified according to the European regulation (EC) No1272/2008 in the hazard class Repr.1B code H360D (may damage the unburned child), Eye Irrit.2 code H319, STOT SE 3 code H335, Skin Irrit.2 H315 and according to the European directive 67/548/EEC it is classified as Reprotoxic Cat2 code R61, Xi codes R36/37/38. Further more, it is submitted to the Toxic Release Inventory (SARA Title III Section 313).